DragDrop: added functionality to drag/drop dataset config files from file dialog

Based on drag/drop functionality and the new dataset config files, this MR enables dropping dataset config files from the file explorer into Met.3D. Currently they have to be dropped into the scene. By default, the add dataset dialog is shown. Using the Alt Modifier when dropping the file, this dialog is skipped. It also refactors the method to create the dataset dialog out of the main window.

Closes #399 (closed)